1. Field
The present disclosure is directed to a method and apparatus for auto exposure value detection for High Dynamic Range (HDR) Imaging. More particularly, the present disclosure is directed to determining optimal exposure values for high and low exposure images to generate an HDR image.
2. Introduction
Presently, exposure determines how much light gets to film or a camera sensor. All still cameras have at least two fundamental controls for this: lens aperture and shutter speed. The combination of the two is the Exposure Value (EV) used for exposure. Exposure Value also has second equivalent definition. The second definition is how much exposure is required by the combination of subject luminance (e.g., how bright it is) and film speed. EV is also called Additive Photographic Exposure (APEX). Some cameras, such as those on smartphones and tablets, cannot change the lens aperture.
Any camera can actually capture a vast dynamic range—just not in a single image. By varying the shutter speed, aperture size, and/or camera sensor gain, which is equivalent to the ISO speed, most digital cameras can change how much light they let in to an image. HDR imaging utilizes this characteristic by creating images composed of multiple exposures to cover a larger dynamic range. Due to a limitation of processing time for most real-time cases on a camera an HDR image is created by using source images acquired at the exposure values: normal exposure, high exposure, and low exposure. An HDR image can also be created using two source images acquired at two exposure values.
Recently, a new kind of HDR imaging is available on a Complementary Metal-oxide Semiconductor (CMOS) camera sensor. On one captured image, half of the pixels of the camera sensor are exposed shorter, called the short exposure pixels and the other half of the pixels are exposed longer, called the long exposure pixels. This feature is equivalent to taking two input images simultaneously and is used to generate an HDR image on the fly. The CMOS sensor industry has used “iHDR” (interlaced or interleaved HDR) or “zigzag HDR” (various mosaic patterns) to describe this new feature.
Unfortunately, all of the current HDR techniques do not provide a method for determining the optimal EV for the high and low exposure images, used to generate an HDR image.